Two important requirements of an electric actuator intended for use in a subsea installation are to minimise the power required for operation and to allow the actuator to be put into a predetermined state (typically one which closes a respective valve) on the occurrence of partial or complete failure of the electrical supply to a motor which drives the actuator. These two requirements may well be in conflict. More particularly, it is currently considered desirable to provide a return spring which is disposed to act to return the actuator to a datum state. A drive system which has to work against the force of the return spring consumes power unnecessarily and it is one object of the electric actuator to include the safety feature of a return (or ‘fail-safe’) spring and to allow normal working of the motor or motors for the actuator free from the action of the return spring.